"More than other hormones, testosterone goes up and down throughout the course of the day, and it's highest between 8 and 10 in the morning," he says. "What this means is you can give someone a 'false' low testosterone level if you measure it in the afternoon."

So the test should be done between 8 and 10 am, Tamler says. And it needs to be done twice, on different days, to confirm results.

If levels are low, Tamler says there are a number of reasons why that could be. "The most common reason for low testosterone in my practice, I find, is obesity," he says. That's because fat tissue, especially abdominal fat, can turn testosterone into the female hormone estradiol.

That transformation has two effects, Tamler says. First, there's just less testosterone in the body. And second, the extra estradiol disrupts signals to the brain that stimulate testosterone production, telling the body not to produce more testosterone.

When obese men had weight loss surgery and lost about 40 pounds, their testosterone levels doubled in just one year, Tamler found in a recent study.

Diabetes and sleep apnea can also cause low testosterone and should be treated first, he says, before going on hormone therapy.

If it turns out that patients have none of these problems and their testosterone levels are truly low, then doses of the supplement need to be carefully tweaked in order to make sure it doesn't cause production of too many red blood cells, which can lead to deadly blood clots.

"The amount of red blood cells in circulation can go up very high, and that can mean the blood does not flow as smoothly as it should and that can give you troubles," Tamler says. Clots can cause heart attacks and strokes.

People on testosterone therapy need to be re-evaluated every six months, Tamler says, to check on red blood cell production, blood pressure and estradiol.

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Transcript

PATTI NEIGHMOND, BYLINE: And I'm Patti Neighmond.

If you're a man and worried about your testosterone levels, endocrinologist Ronald Tamler says you should get a blood test to measure levels first thing in the morning.

DR. RONALD TAMLER: More than other hormones, testosterone goes up and down throughout the course of the day and its highest between 8 and 10 in the morning.

DR. BILL REILLY: If the hormone is measured in the afternoon, results can be misleading. They may be low when in fact they're just low in the afternoon. And you should have the test done twice on different days just to be sure. If it truly is low, there are a number of possible causes.

TAMLER: The most common reason for low testosterone in my practice I find is obesity.

TAMLER: And that has two affects; the first one is that there's less testosterone.

NEIGHMOND: The second one is that the extra estradiol disrupts signals to the brain, telling it not to produce more testosterone.

In a recent study, Tamler found when obese men had weight loss surgery and lost on average about 40 pounds, their testosterone levels doubled over the course of a year. Diabetes and sleep apnea can also cause low testosterone and should be treated first, he says, before embarking on hormone therapy.

If it turns out testosterone levels are truly low, then doses of the supplement need to be carefully tweaked to make sure it doesn't cause production of too many red blood cells.

TAMLER: The amount of red blood cells in circulation can go up very high, and that can mean that the blood does not flow as smoothly as it should and that can give you troubles. You don't want to have blood that is too thick because it can cause clots.

NEIGHMOND: Leading to heart attack or stroke. Tamler says its important for patients to see a specialist once every six months.

Patti Neighmond, NPR News.

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DAVID GREENE, HOST:

The first piece in our health segment from Sarah Varney came to us from our partner Kaiser Health News, a nonprofit news service.